The present invention relates to a device for pressing a substrate cutting tool, in particular a mostly circular knife, against another device interacting with the tool. The device is used to machine substrates made of continuous webs or sheets of paper, film, fabric, metals, plastics or the like.
Devices of the aforementioned kind are usually used to cut, perforate or likewise machine relatively wide webs in their longitudinal direction, for example, into significantly narrower strips. After the machining operation, the resulting strips travel to another processing machine. The cutting tool that is used interacts with a matching counter-tool. One example is a cutting tool designed as an upper knife which has a substantially circular outer contour and a corresponding bottom knife. However, it is also possible that the substrate to be machined, for example a web of paper, loops around a rotatable, revolving cylinder, where at least one upper knife presses the substrate against the cylinder so as to machine the substrate. During the respective machining operation, it is necessary that the tools used in the machining operation interact well with one another so that a desired result of the machining operation with desired quality can be obtained.
During what is known as a scissor cut, which is obtained with rotary knives, for example, with an upper and a bottom rotary knife, interaction of the knives is crucial. If the interacting knives do not interact well with one another during the cutting operation unclean cut edges results. This result can lead to difficulties in the subsequent winding up operation of the cut strips. Other difficulties arise when cutting such substrates at higher travel velocities of the substrates, i.e. as the speed of the usually rotatable cutting tool increases. Often the causes for this are irregularities in the quality of the material of the participating tools. Furthermore, it is desirable to have one cutting tool with which a wide range of materials can be machined, for example, thin and thick webs of paper or thin and thick webs of plastic film.
A cutter with upper and bottom knives is known from U.S. Pat. No. 3,055,249, wherein, for example, the upper knife is arched so that it is elastic and thus can be pressed against the respective bottom knife. With this device the force with which the respective upper knife presses against the respective bottom knife can be set only relatively coarsely, i.e., with the aid of the elasticity of the upper knife. It is not possible to adjust the force to the substrate to be cut.
A device is known from German Patent 310 796 in which the upper knife is pressed against the bottom knife with the aid of a spring. Thus, it is possible through the use of selectively different springs to press the upper knife with varying strength against the bottom knife. The drawback here is that the respective spring rubs against the upper knife and the frictional forces are subject, as well known, to a stick-slip effect, i.e., the transfer of frictional forces that cannot be precisely defined. Thus, it is difficult to transfer the pressure forces to the appropriate active point. This applies especially when the rotary knife is rotated at a very high speed about its geometric axis, such high speed results in very rapidly changing forces at the cutting point, for example, cutting forces and bending of the rotary knife. Thus, constantly changing frictional forces of the spring results, especially when the substrates are machined at a fast travel speed. The same applies to devices as known from German utility model 18 41 256, in which the knives ar pressed against the appropriate counterknife by loop springs.
Another device is known from German Patent 697 108, in which the knife is positioned on an auxiliary hub, where the auxiliary hub can be pivoted with respect to a main hub by spring forces. This device also has relatively large interacting tools rendering it difficult to machine a substrate evenly at today's high travel speeds which subject the device to knife bending and expansion when passing the cutting point. Thus, poor cutting quality results.